193. Wrong

You have garbled the report. First most of the guns in Central America come from the US since we waged war on the region. True, the Russians supplied some weapons to the guerillas via Cuba, but not nearly as many as we did.

Now for the report. 87% of the weapons that could be traced came through the US. Even most of those manufactured outside of the US were shown to have been trafficked through the US.

There has clearly been a game of telephone going on among gun proponents regarding this report. You all have taken the arms not traceable and falsely concluded that they did not come from the US, when there is no evidence to suggest that. In fact the data from the traceable guns is a large sample and can be seen as representative of the overall composition of guns trafficked into Mexico.

I am excerpting part of the report here and will link to the original so you can read it yourself.

Using ATF’s eTrace data, which currently serves as the best data we found
available for analyzing the source and nature of firearms trafficked and
seized in Mexico, we determined over 20,000, or 87 percent, of firearms
seized by Mexican authorities and traced from fiscal year 2004 to fiscal
year 2008 originated in the United States. Figure 3 shows the percentages
of firearms seized in Mexico and traced from fiscal year 2004 to fiscal year
2008 that originated in the United States. Over 90 percent of the firearms
seized in Mexico and traced over the last 3 years have come from the
United States.

Around 68 percent of these firearms were manufactured in the United
States, while around 19 percent were manufactured in third countries and
imported into the United States before being trafficked into Mexico. ATF
could not determine whether the remaining 13 percent foreign sourced
arms had been trafficked into Mexico through the United States, due to
incomplete information.

As for the Mexican end, GAO reports:

While the eTrace data only represents data from gun trace requests
submitted from seizures in Mexico and not all the guns seized, it is
currently the only systematic data available, and the conclusions from its
use that the majority of firearms seized and traced originated in the United
States were consistent with conclusions reached by U.S. and Mexican
government and law enforcement officials involved personally in
combating arms trafficking to Mexico. In 2008, of the almost 30,000
firearms that the Mexican Attorney General’s office said were seized, only
around 7,200, or approximately a quarter, were submitted to ATF for
tracing. U.S. and Mexican government and law enforcement officials
indicated Mexican government officials had not submitted all of the
firearms tracing information due to bureaucratic obstacles between the
Mexican military and the Mexican Attorney General’s Office and lack of a
sufficient number of trained staff to use eTrace. For instance, at one point,
State officials told us, the Government of Mexico had only one staff person
collecting gun information and entering it into eTrace.12 Further, as ATF
pointed out, not all guns seized in the United States are submitted by U.S.
entities to ATF for tracing either, due to some of the same type of
bureaucratic and resource challenges faced in Mexico. Consistent with the
results of eTrace data, U.S. law enforcement officials who had worked on
arms trafficking in Mexico and along the U.S.-Mexican border told us their
experience and observations corroborated that most of the firearms in
Mexico had originated in the United States. Furthermore, U.S. and
Mexican government and law enforcement officials also stated this
scenario seemed most likely, given the ease of acquiring firearms in the
United States; specifically, they told us they saw no reason why the drug
cartels would go through the difficulty of acquiring a gun somewhere else
in the world and transporting it to Mexico when it is so easy for them to do
so from the United States.

The go on to list the specific types of weapons and how they have been altered to make them more lethal.

It shows maps of how the guns flow from various parts of the US into Mexico.

Conclusions:

Combating arms trafficking has become an increasing concern to U.S. and
Mexican government and law enforcement officials, as violence in Mexico
has soared to historic levels, and U.S. officials have become concerned
about the potential for increased violence brought about by Mexican DTOs
on the U.S. side of the border. However, while this violence has raised
concern, there has not been a coordinated U.S. government effort to
combat the illicit arms trafficking to Mexico that U.S. and Mexican
government officials agree is fueling much of the drug-related violence.

Agencies such as ATF and ICE have made some efforts to combat illicit
arms trafficking, but these efforts are hampered by a number of factors,
including the constraints of the legal framework in which law enforcement
agencies operate, according to agency officials, and poor coordination
among agencies. In addition, agencies have not systematically and
consistently gathered and reported certain types of data on firearms
trafficking that would be useful to the administration and Congress to
better target resources to combat arms trafficking to Mexico. Gaps in this
data hamper the investigative capacity of law enforcement agencies.
Further, a Spanish language version of ATF’s eTrace has been in
development for months but has yet to be finalized; the lack of this new
version of eTrace has impeded the use of eTrace by Mexican law
enforcement officials, which limits data that could be used in
investigations on both sides of the border and results in incomplete
information on the nature of firearms trafficked and seized in Mexico.
Quick deployment of eTrace across Mexico and training of the relevant
officials in its use could increase the number of guns submitted to ATF for
tracing each year, improving the data on the types and sources of firearms
trafficked into Mexico and increasing the information that law
enforcement officials have to investigate and build cases.

U.S. and Mexican government officials in locations we visited told us that,
while they have undertaken some efforts to combat illicit arms trafficking,
they are concerned that without a targeted, comprehensive, and
coordinated U.S. government effort, their efforts could fall short. In June
2009, the administration released its 2009 National Southwest Border
Counternarcotics Strategy, containing a chapter on arms trafficking to
Mexico. We reviewed the strategy’s chapter on arms trafficking and found
that the chapter does contain some key elements of a strategy, such as
setting objectives, but it lacks others, such as providing detailed roles and
responsibilities for relevant agencies or performance measures for
monitoring progress toward objectives. ONDCP officials said they will
develop an implementation plan for the strategy in late summer of 2009
that will have more detailed actions for each agency to take, as well as
some performance measures for each item under the objectives. However,
at this point, it is not clear whether the implementation plan will include
performance indicators and other accountability mechanisms to overcome
shortcomings raised in our report. Furthermore, in March 2009, the
administration announced more resources for the Southwest border,
including more personnel and equipment for conducting southbound
inspections. However, it is unclear how the new resources that the
administration has recently devoted to the Southwest border will be tied
to the new strategy and implementation plan.

The current level of cooperation on law enforcement issues between the
United States and Mexico under President Calderon’s administration
presents a unique opportunity to work jointly to combat illicit arms
trafficking. Taking advantage of this opportunity will require a unified,
U.S. government approach that brings to bear all the necessary assets to
combat illicit arms trafficking.

Now, if the US government could absolve itself of responsibility for gun trafficking into Mexico, don't you think they would? The government is not in the habit of creating more diplomatic hassles without some driving national interest. There is no national interest in claiming more guns flow from Mexico into the US than actually do.
So time to readjust your argument. I would suggest you simply leave Mexico off the rhetorical menu. It does not help your case and only shows how badly the gun side distorts evidence.